Before there was inkjet decal paper, there was laser decal paper, which would work with either laser printers or laser copiers, and I used to make decals by printing them out on regular paper and then take them to a copy shop to be copied to the decal paper, which I brought along and supplied. But, different copiers run at different temps, and I had to do test runs at various copy shops before I found one that ran at a low enough temp, and he soon was out of business. However, fortunately, they had perfected the inkjet decal paper by then.
So now you can copy kit decals, and reproduce them on your inkjet printer. However (and this was true also on laser systems), the printers do not print white, so you have a problem with decals containing white or very light colors. Dark and black decals are easy.
For white or light colored decals, you have to use white decal paper- the whole sheet is white. This means you either have to cut the decal out exactly to remove the white fringe, or set the background color (if you are doing your own graphics) to the color you will be putting the decal down over- that is not a trivial task. As I say, this also applies to laser-copied decals. The copier will print white areas as clear.